Mass analysis, and more particularly mass spectrometry, has proven to be an effective analytical technique for identifying unknown compounds and for determining the precise mass of known compounds. Advantageously, compounds can be detected or analysed in minute quantities allowing compounds to be identified at very low concentrations in chemically complex mixtures. Not surprisingly, mass spectrometry has found practical application in medicine, pharmacology, food sciences, semi-conductor manufacturing, environmental sciences, security, and many other fields.
A typical mass spectrometer includes an ion source that ionizes particles of interest. The ions are passed to an analyser region, where they are separated according to their mass (m)-to-charge (z) ratios (m/z). The separated ions are detected at a detector. A signal from the detector may be sent to a computing or similar device where the m/z ratios may be stored together with their relative abundance for presentation in the format of a m/z spectrum.
Typical ion sources are detailed in “Ionization Methods in Organic Mass Spectrometry”, Alison E. Ashcroft, The Royal Society of Chemistry, UK, 1997; and the references cited therein. Conventional ion sources may, for example, create ions by electrospray or chemical ionization.
Electrospray ionization involves dispersing liquid containing analyte(s) of interest into a fine aerosol jet of solvated charged droplets. Typically, a nebulizer gas flow is involved in this dispensing process and an impinging heater gas flow assists droplet desolvation. Charged droplets are drawn by an electric field to the sampling inlet of a mass spectrometer. Liquid flows greater than 25 μL/m usually require the various gas flows to be heated for rapid desolvation.
Atmospheric pressure chemical ionization (“APCI”) relies on liquid containing analyte of interest to be discharged into a fine aerosol jet of droplets containing the analyte. Again, a nebulizer gas flow is involved and an impinging heater gas flow may assist droplet desolvation. Desolvated analyte molecules are chemically ionized by reagent ions created in close proximity by a corona current.
It has long been recognized that the sampling inlet is a major sensitivity bottleneck: typical diameters of the sampling inlet are about 0.5 mm, and space repulsion of analyte ions acts as a choke upon significant sensitivity increases. Although larger sampling diameters are desired for higher sensitivity, such apertures necessitate larger vacuum pumps. Present vacuum pumping systems are at their practical maximum in terms of size and cost.
Accordingly, alternative approaches are required.